NOx absorbent and absorption reduction-type NOx purifying catalyst

ABSTRACT

An exhaust gas purifying catalyst capable of exerting excellent NO x  purifying capability even at high exhaust gas temperatures, exceeding 500° C., is provided. 
     The catalyst is an absorption reduction-type NO x  purifying catalyst comprising an NO x  absorbent obtained by loading at least one alkali metal or alkaline earth metal on a crystalline ZrO 2  composite oxide having added thereto at least one trivalent rare earth metal or divalent alkaline earth metal. Preferably, aluminum is further supported on the ZrO 2  composite oxide. The NO x  absorbent is considered to have an NO x  absorption mechanism that an electron released from the alkali metal or the like enters into an oxygen vacancy part of the crystalline ZrO 2  and this oxygen vacancy part functions as a strong base site for donating an electron to an electrophilic component such as NO x .

TECHNICAL FIELD

The present invention relates to a novel NO_(x) absorbent which is used in a catalyst for purifying an exhaust gas discharged from an automobile engine and the like and can remarkably improve the NO_(x) purifying capability at high temperatures, and also relates to an absorption reduction-type NO_(x) purifying catalyst.

BACKGROUND ART

Recently, from the standpoint of global conservation, it is a worldwide problem to suppress the total amount of carbon dioxide (CO₂) exhausted from an internal combustion engine such as automobile engine and the generated amount of nitrogen oxide (NO_(x)). In order to solve this problem, a lean-burn engine has been developed for the purpose of improving the fuel consumption and an absorption reduction-type NO_(x) purifying catalyst, obtained by adding a function of occluding NO_(x) in a lean atmosphere to a conventional three-way catalyst, has been developed for the purpose of purifying the exhaust gas of the lean-burn engine. These technologies are having some success against the above-described problem.

In the lean-burn engine combined with the absorption reduction-type NO_(x), purifying catalyst, the fuel is usually burned in a lean (oxygen-excess) condition and temporarily burned in a stoichiometric (at a theoretical air-fuel ratio) or rich (fuel-excess) condition.

HC (hydrocarbon) or CO in the exhaust gas is efficiently burned and removed in the lean condition by the action of catalyst using the oxidative atmosphere. On the other hand, NO_(x) is captured by an absorbent in the lean condition and this is temporarily released in the stoichiometric or rich condition and reduced and purified by the action of catalyst using the reducing atmosphere.

By virtue of these combustion conditions and actions of the absorption reduction-type NO_(x) purifying catalyst, as a whole, the fuel consumption is improved and, at the same time, HC, CO and NO_(x) in the exhaust gas can be purified with good efficiency.

In this absorption reduction-type NO_(x) purifying catalyst, a noble metal such as Pt, Au, Pd and Rh is used as the catalyst component and an alkali metal such as K and Na or an alkaline earth metal such as Ca and Ba is used as the NO_(x) absorbent.

This lean-burn system established by combining the control of an air-fuel ratio and the NO_(x) absorbent is successful to a certain extent in solving the problem to improve the fuel consumption and reduce the total generation amount of CO, HC and NO_(x) as compared with the conventional system using a three-way catalyst and combustion in the vicinity of theoretical air-fuel ratio.

Japanese Unexamined Patent Publication (Kokai) No. 9-24274 describes an exhaust gas purifying catalyst comprising a composite oxide containing alumina and zirconia and having an acid point and a base point, and Kokai No. 11-226404 describes a stabilized zirconia support for improving the steam reforming reactivity of catalyst component Rh.

However, the absorption reduction-type NO_(x) purifying catalyst using the above-described alkali metal or alkaline earth metal as the NO_(x) absorbent has a problem that the NO_(x) purifying capability decreases when the exhaust gas temperature is high and exceeds about 500° C.

This is thought to occur because, although the alkali metal or alkaline earth metal exerts the function as NO_(x) absorbent by absorbing NO_(x) while forming a nitrate in the lean side and by releasing NO_(x) while forming a carbonate in the rich side, the nitrate or the like of the alkali metal or alkaline earth metal decomposes at a high temperature exceeding about 500° C. and, therefore, the intended function cannot be brought out.

Accordingly, an object of the present invention is to provide an exhaust gas purifying catalyst capable of exerting high NO_(x) purifying capability even at a high temperature exceeding about 500° C. by using, as the NO_(x) absorbent, a specific modified ZrO₂ composite oxide, different from those in conventional techniques, in place of an alkali metal or an alkaline earth metal.

DISCLOSURE OF THE INVENTION

The object of the present invention can be attained by an NO_(x) absorbent comprising a crystalline ZrO₂ composite oxide having supported thereon at least one alkali metal or alkaline earth metal, the composite oxide being added with at least one element selected from trivalent rare earth metals and divalent alkaline earth metals, and also by an absorption reduction-type NO_(x) purifying catalyst comprising this NO_(x) absorbent.

More specifically, the present invention is characterized by using a specific modified crystalline ZrO₂ composite oxide as the NO_(x) absorbent. The crystalline ZrO₂ composite oxide is considered to have the following NO_(x) absorption mechanism.

Zr constituting the crystalline ZrO₂ composite oxide is tetravalent and therefore, when a trivalent rare earth metal or a divalent alkaline earth metal is added, an oxygen vacancy is generated in the crystalline ZrO₂ composite oxide. When an alkali metal or an alkaline earth metal is subsequently supported thereon, an electron released from the alkali metal or alkaline earth metal enters into the oxygen vacancy part. This electron is bound by a constituent element in the structure of crystalline ZrO₂ composite oxide but the binding degree is low because the electron is present in the oxygen vacancy part, therefore, the electron can be readily bonded to an electrophilic component such as NO_(x), particularly NO₂. In other words, the crystalline ZrO₂ composite oxide can be rich in “strong base sites” which readily donate electrons to electrophilic components.

As the ZrO₂ composite oxide is crystalline, the supported alkali metal or alkaline earth metal stays in the vicinity of surface of the crystalline ZrO₂ composite oxide and, therefore, the strong base site is present in the vicinity of surface of the crystalline ZrO₂ composite oxide and can effectively act on NO_(x), particularly NO₂, in the exhaust gas. Incidentally, it has been verified by the present inventors that, if the ZrO₂ composite oxide is non-crystalline, the alkali metal or alkaline earth metal supported enters inside the ZrO₂ composite oxide and the strong base site is not produced.

The crystalline ZrO₂ composite oxide having a strong base site in the vicinity of surface, as such, donates an electron less bound by a constituent element to NO₂ in the exhaust gas and thereby NO₂ can have a negative electric charge. At the same time, the crystalline ZrO₂ composite oxide becomes to have a positive electric charge as a result of the electron donation. By the attractive force between these electric charges, the crystalline ZrO₂ composite oxide is presumed to absorb NO₂ as the negative ion.

It is considered that the crystalline ZrO₂ composite oxide has a large NO₂ ion absorbing energy due to the attractive force between electric charges and thereby can exert the function of an NO_(x) absorbent using the above-described absorption mechanism even at a temperature in excess of 500° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph comparing the NO_(x) purification percentage of the exhaust gas purifying catalyst of the present invention at high temperatures by varying the component added.

FIG. 2 is a graph comparing the NO_(x) purification percentage of the exhaust gas purifying catalyst of the present invention at high temperatures by varying the component added.

FIG. 3 is a graph comparing the NO_(x) purification percentage of the exhaust gas purifying catalyst of the present invention at high temperatures by varying the component added.

FIG. 4 is a graph comparing the NO_(x) purification percentage of the exhaust gas purifying catalyst of the present invention at high temperatures by varying the component added.

FIG. 5 is a graph comparing the NO_(x) purification percentage of the exhaust gas purifying catalyst of the present invention at high temperatures by varying the component added.

FIG. 6 is a graph comparing the NO_(x) purification percentage of the exhaust gas purifying catalyst of the present invention at high temperatures by varying the component added.

FIG. 7 is a graph comparing the NO_(x) purification percentage of exhaust gas purifying catalysts.

FIG. 8 is graph comparing the NO_(x) purification percentage of exhaust gas purifying catalysts after an endurance heat treatment.

BEST MODE FOR CARRYING OUT THE INVENTION

The NO_(x) absorbent of the present invention comprises a crystalline ZrO₂ composite oxide having supported thereon at least one alkali metal or alkaline earth metal, the composite oxide being added with at least one trivalent rare earth metal or divalent alkaline earth metal.

Examples of the trivalent rare earth metal include Sc, Y, La, Nd, Pm, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu. Examples of the divalent alkaline earth metal include Mg, Ca, Sr, Ba and Ra. In the NO_(x) absorbent of the present invention, a crystalline ZrO₂ composite oxide having an oxygen vacancy formed by adding at least one element of these trivalent rare earth metals and divalent alkaline earth metals to ZrO₂ is used.

For example, an aqueous solution of a nitrate, an acetate, a carbonate or the like of the above-described trivalent rare earth metal or divalent alkaline earth metal is impregnated into ZrO₂ powder and this powder is dried and calcined in an air atmosphere at 700 to 1,200° C. to solid-dissolve the trivalent rare earth metal or the like in ZrO₂, whereby the crystalline ZrO₂ composite oxide having an oxygen vacancy can be obtained.

In the obtained crystalline ZrO₂ composite oxide having an oxygen vacancy, when represented by the chemical structural formula: M_(m)Zr_(l-m)O_(n) preferably 0.01<m<0.5, more preferably 0.1<m<0.2, and 1.5<n<2.0, more preferably 1.8<n<1.95.

On this crystalline ZrO₂ composite oxide having an oxygen vacancy, at least one alkali metal or alkaline earth metal is supported. Examples of the alkali metal include Li, Na, K, Rb, Cs and Fr, and examples of the alkaline earth metal include Mg, Ca, Sr, Ba and Ra.

For example, the ZrO₂ composite oxide is impregnated with an aqueous solution of a nitrate, an acetate, a carbonate or the like of this alkali metal or alkaline earth metal, then dried and calcined in an air atmosphere at 400 to 800° C., whereby the alkali metal or alkaline earth metal can be supported on the crystalline ZrO₂ composite oxide having an oxygen vacancy.

By this loading, an electron released from the alkali metal or alkaline earth metal is presumed to enter into the oxygen vacancy part of the crystalline ZrO₂ composite oxide having an oxygen vacancy, thereby producing a strong base site.

The amount of the alkali metal or alkaline earth metal supported is preferably from 0.01 to 0.3 mol, more preferably from 0.03 to 0.1 mol, per mol of Zr.

It is found that the capability of the NO_(x) absorbent can be more enhanced by further loading Al on the crystalline ZrO₂. The reason therefor is considered to be because the previously loaded alkali metal or alkaline earth metal is stabilized and high dispersibility of alkali metal or the like is attained.

For loading Al, for example, the crystalline ZrO₂ composite oxide is impregnated with a solution of aluminum nitrate, Al(OCH(CH₃)₂)₃ or the like, then dried and calcined in an air atmosphere at 400 to 800° C.

The amount of Al supported is preferably 1 mol or less, more preferably from 0.5 to 0.001 mol, still more preferably from 0.2 to 0.001 mol, per mol of Zr.

The absorption reduction-type NO_(x) purifying catalyst of the present invention comprises a catalyst component of accelerating the oxidation/reduction of NO_(x). This catalyst component is preferably a noble metal such as Pt, Au, Ru, Rh, Pd and Ir.

The catalyst component may be supported on an oxide support such as alumina, zirconia, titania and silica-alumina and be present together with the NO_(x) absorbent, or may be directly supported on the NO_(x) absorbent.

The method for loading the catalyst component can be selected from a wide range of methods, for example, evaporation to dryness, precipitation, adsorption, ion exchange and reduction-deposition.

EXAMPLES Example 1

Scandium nitrate was added to ZrO₂ powder and this powder was calcined in an air at about 1,000° C. and then pulverized to obtain a solid solution powder with an oxygen vacancy having a composition of Sc_(0.11)Zr_(0.89)O_(1.945).

Thereafter, 120 g of this powder having a composition of Sc_(0.11)Zr_(0.89)O_(1.945) was impregnated with an aqueous sodium nitrate solution, then dried in an air at 120° C. for one hour and calcined in an air at 500° C.

The resulting powder was calcined in an air at 850° C. for 2 hours to stabilize Na and, at the same time, dissipate unstable Na not taken in into the solid solution, thereby obtaining an NO_(x) absorbent having a strong base point. The obtained NO_(x) absorbent had a composition of Sc_(0.11)Zr_(0.89)O_(1.945)/Na_(0.2).

Separately, 120 g of γ-Al₂O₃ powder was impregnated with an aqueous dinitrodiammine platinum solution in an amount corresponding to 2 g of Pt and rhodium nitrate in an amount corresponding to 0.1 g of Rh, then dried in air at 120° C. for one hour and calcined in air at 500° C. for one hour to obtain γ-Al₂O₃ having supported thereon Pt and Rh as catalyst components (hereinafter referred to as “Pt supported γ-Al₂O₃”).

The NO_(x) absorbent was mixed with the Pt supported γ-Al₂O₃ in an equivalent mass and the mixture was pelletized to obtain a catalyst of the present invention.

Example 2

An NO_(x) absorbent having a composition of Sc_(0.11)Zr_(0.89)O_(1.945)/K_(0.2) was prepared in the same manner as in Example 1 except for using an aqueous potassium nitrate solution in place of the aqueous sodium nitrate solution of Example 1.

The obtained NO_(x) absorbent was mixed with the Pt supported γ-Al₂O₃ in an equivalent mass similarly to Example 1 and the mixture was pelletized to obtain a catalyst of the present invention.

Example 3

An NO_(x) absorbent having a composition of Sc_(0.11)Zr_(0.89)O_(1.945)/Cs_(0.2) was prepared in the same manner as in Example 1 except for using an aqueous cesium nitrate solution in place of the aqueous sodium nitrate solution of Example 1.

The obtained NO_(x) absorbent was mixed with the Pt supported γ-Al₂O₃ in an equivalent mass similarly to Example 1 and the mixture was pelletized to obtain a catalyst of the present invention.

Example 4

An NO_(x) absorbent having a composition of Sc_(0.11)Zr_(0.89)O_(1.945)/Ba_(0.2) was prepared in the same manner as in Example 1 except for using an aqueous barium acetate solution in place of the aqueous sodium nitrate solution of Example 1.

The obtained NO_(x) absorbent was mixed with the Pt supported γ-Al₂O₃ in an equivalent mass similarly to Example 1 and the mixture was pelletized to obtain a catalyst of the present invention.

Example 5

An NO_(x) absorbent having a composition of Y_(0.07)Zr_(0.93)O_(1.965)/Na_(0.2) was prepared in the same manner as in Example 1 except for using yttrium nitrate in place of the scandium nitrate of Example 1. The obtained NO_(x) absorbent was mixed with the Pt supported γ-Al₂O₃ in an equivalent mass similarly to Example 1 and the mixture was pelletized to obtain a catalyst of the present invention.

Example 6

An NO_(x) absorbent having a composition of Y_(0.07)Zr_(0.93)O_(1.965)/K_(0.2) was prepared in the same manner as in Example 2 except for using yttrium nitrate in place of the scandium nitrate of Example 2. The obtained NO_(x) absorbent was mixed with the Pt supported γ-Al₂O₃ prepared in an equivalent mass similarly to Example 2 and the mixture was pelletized to obtain a catalyst of the present invention.

Example 7

An NO_(x) absorbent having a composition of Y_(0.07)Zr_(0.93)O_(1.965)/Cs_(0.2) was prepared in the same manner as in Example 3 except for using yttrium nitrate in place of the scandium nitrate of Example 3. The obtained NO_(x) absorbent was mixed with the Pt supported γ-Al₂O₃ in an equivalent mass similarly to Example 3 and the mixture was pelletized to obtain a catalyst of the present invention.

Example 8

An NO_(x) absorbent having a composition of Y_(0.07)Zr_(0.93)O_(1.965)/Ba_(0.2) was prepared in the same manner as in Example 4 except for using yttrium nitrate in place of the scandium nitrate of Example 4. The obtained NO_(x) absorbent was mixed with the Pt supported γ-Al₂O₃ in an equivalent mass similarly to Example 4 and the mixture was pelletized to obtain a catalyst of the present invention.

Example 9

An NO_(x) absorbent having a composition of La_(0.2)Zr_(0.8)O_(1.9)/Na_(0.2) was prepared in the same manner as in Example 1 except for using lanthanum nitrate in place of the scandium nitrate of Example 1. The obtained NO_(x) absorbent was mixed with the Pt supported γ-Al₂O₃ in an equivalent mass similarly to Example 1 and the mixture was pelletized to obtain a catalyst of the present invention.

Example 10

An NO_(x) absorbent having a composition of La_(0.2)Zr_(0.8)O_(1.9)/K_(0.2) was prepared in the same manner as in Example 2 except for using lanthanum nitrate in place of the scandium nitrate of Example 2. The obtained NO_(x) absorbent was mixed with the Pt supported γ-Al₂O₃ in an equivalent mass similarly to Example 2 and the mixture was pelletized to obtain a catalyst of the present invention.

Example 11

An NO_(x) absorbent having a composition of La_(0.2)Zr_(0.8)O_(1.9)/Cs_(0.2) was prepared in the same manner as in Example 3 except for using lanthanum nitrate in place of the scandium nitrate of Example 3. The obtained NO_(x) absorbent was mixed with the Pt supported γ-Al₂O₃ in an equivalent mass similarly to Example 3 and the mixture was pelletized to obtain a catalyst of the present invention.

Example 12

An NO_(x) absorbent having a composition of La_(0.2)Zr_(0.8)O_(1.9)/Ba_(0.2) was prepared in the same manner as in Example 4 except for using lanthanum nitrate in place of the scandium nitrate of Example 4. The obtained NO_(x) absorbent was mixed with the Pt supported γ-Al₂O₃ in an equivalent mass similarly to Example 4 and the mixture was pelletized to obtain a catalyst of the present invention.

Example 13

An NO_(x) absorbent having a composition of Mg_(0.12)Zr_(0.88)O_(1.94)/Na_(0.2) was prepared in the same manner as in Example 1 except for using magnesium nitrate in place of the scandium nitrate of Example 1. The obtained NO_(x) absorbent was mixed with the Pt supported γ-Al₂O₃ in an equivalent mass similarly to Example 1 and the mixture was pelletized to obtain a catalyst of the present invention.

Example 14

An NO_(x) absorbent having a composition of Mg_(0.12)Zr_(0.88)O_(1.94)/K_(0.2) was prepared in the same manner as in Example 2 except for using magnesium nitrate in place of the scandium nitrate of Example 2. The obtained NO_(x) absorbent was mixed with the Pt supported γ-Al₂O₃ in an equivalent mass similarly to Example 2 and the mixture was pelletized to obtain a catalyst of the present invention.

Example 15

An NO_(x) absorbent having a composition of Mg_(0.12)Zr_(0.88)O_(1.94)/Cs_(0.2) was prepared in the same manner as in Example 3 except for using magnesium nitrate in place of the scandium nitrate of Example 3. The obtained NO_(x) absorbent was mixed with the Pt supported γ-Al₂O₃ in an equivalent mass similarly to Example 3 and the mixture was pelletized to obtain a catalyst of the present invention.

Example 16

An NO_(x) absorbent having a composition of Mg_(0.12)Zr_(0.88)O_(1.94)/Ba_(0.2) was prepared in the same manner as in Example 4 except for using magnesium nitrate in place of the scandium nitrate of Example 4. The obtained NO_(x) absorbent was mixed with the Pt supported γ-Al₂O₃ in an equivalent mass similarly to Example 4 and the mixture was pelletized to obtain a catalyst of the present invention.

Example 17

An NO_(x) absorbent having a composition of Ca_(0.14)Zr_(0.86)O_(1.93)/Na_(0.2) was prepared in the same manner as in Example 1 except for using calcium nitrate in place of the scandium nitrate of Example 1. The obtained NO_(x) absorbent was mixed with the Pt supported γ-Al₂O₃ in an equivalent mass similarly to Example 1 and the mixture was pelletized to obtain a catalyst of the present invention.

Example 18

An NO_(x) absorbent having a composition of Ca_(0.14)Zr_(0.86)O_(1.93)/K_(0.2) was prepared in the same manner as in Example 2 except for using calcium nitrate in place of the scandium nitrate of Example 2. The obtained NO_(x) absorbent was mixed with the Pt supported γ-Al₂O₃ in an equivalent mass similarly to Example 2 and the mixture was pelletized to obtain a catalyst of the present invention.

Example 19

An NO_(x) absorbent having a composition of Ca_(0.14)Zr_(0.86)O_(1.93)/Cs_(0.2) was prepared in the same manner as in Example 3 except for using calcium nitrate in place of the scandium nitrate of Example 3. The obtained NO_(x) absorbent was mixed with the Pt supported γ-Al₂O₃ in an equivalent mass similarly to Example 3 and the mixture was pelletized to obtain a catalyst of the present invention.

Example 20

An NO_(x) absorbent having a composition of Ca_(0.14)Zr_(0.86)O_(1.93)/Ba_(0.2) was prepared in the same manner as in Example 4 except for using calcium nitrate in place of the scandium nitrate of Example 4. The obtained NO_(x) absorbent was mixed with the Pt supported γ-Al₂O₃ in an equivalent mass similarly to Example 4 and the mixture was pelletized to obtain a catalyst of the present invention.

Example 21

This Example shows a case where aluminum is further supported on the composite oxide.

The solid solution powder with an oxygen vacancy having a composition of La_(0.2)Zr_(0.8)O_(1.9) used in Example 9 was impregnated with an aqueous aluminum nitrate solution, then dried in an air at 120° C. for one hour and calcined in an air at 650° C. for one hour to obtain a composite oxide having supported thereon 0.005 mol of Al₂O₃ per mol of the composite oxide La_(0.2)Zr_(0.8)O_(1.9).

On this composite oxide, Cs was supported in the same manner as in Example 3 to prepare an NO_(x) absorbent having a composition of La_(0.2)Zr_(0.8)O_(1.9)/(Al₂O₃)_(0.05)/Cs_(0.2). The obtained NO_(x) absorbent was mixed with the Pt supported γ-Al₂O₃ in an equivalent mass similarly to Example 1 and the mixture was pelletized to obtain a catalyst of the present invention.

Example 22

This Example shows a case where aluminum is further supported on the composite oxide and Pt and Cs are supported thereon.

The solid solution powder with an oxygen vacancy having a composition of La_(0.2)Zr_(0.8)O_(1.9) used in Example 9 was impregnated with an aqueous aluminum nitrate solution, then dried in an air at 120° C. for one hour and calcined in an air at 650° C. for one hour to obtain a composite oxide having supported thereon 0.005 mol of Al₂O₃ per mol of the composite oxide La_(0.2)Zr_(0.8)O_(1.9).

Subsequently, 120 g of the obtained composite oxide was impregnated with an aqueous dinitrodiammine platinum solution in an amount corresponding to 2 g of Pt, then dried in an air at 120° C. for one hour and calcined in an air at 500° C. for one hour, thereby loading Pt as the catalyst component.

Thereafter, 0.2 mol of Cs was loaded per 120 g of the composite oxide in the same manner as in Example 3 per 120 g of the composite oxide. Then, γ-Al₂O₃ in an equivalent mass was mixed and the mixture was pelletized to obtain a catalyst of the present invention.

Comparative Example 1

120 g of γ-Al₂O₃ powder was impregnated with an aqueous dinitrodiammine platinum solution in an amount corresponding to 2 g of Pt and rhodium nitrate in an amount corresponding to 0.1 g of Rh, then dried in an air at 120° C. for one hour and calcined in an air at 500° C. for one hour, thereby loading Pt and Rh as the catalyst components.

Subsequently, the resulting powder was impregnated with an aqueous Na nitrate solution, then dried in an air at 120° C. for one hour and calcined in an air at 500° C. for one hour to load 0.2 mol of Na per 120 g of γ-Al₂O₃ powder. Thereafter, γ-Al₂O₃ in an equivalent mass was mixed and the mixture was pelletized to obtain a comparative catalyst.

Comparative Example 2

A comparative catalyst where Pt, Rh and K were supported on γ-Al₂O₃ powder was obtained in the same manner as in Comparative Example 1 except for using an aqueous potassium nitrate solution in place of the aqueous sodium nitrate solution.

Comparative Example 3

A comparative catalyst where Pt, Rh and Cs were supported on γ-Al₂O₃ powder was obtained in the same manner as in Comparative Example 1 except for using an aqueous cesium nitrate solution in place of the aqueous sodium nitrate solution.

Comparative Example 4

A comparative catalyst where Pt, Rh and Ba were supported on γ-Al₂O₃ powder was obtained in the same manner as in Comparative Example 1 except for using an aqueous barium nitrate solution in place of the aqueous sodium nitrate solution.

-Evaluation of NO_(x) Purification Percentage-

(1) NO_(x) Purification Percentage at 500 to 700° C.

The catalysts of Examples 1 to 20 and Comparative Examples 1 to 4 each was evaluated on the NO_(x) purification percentage at 500° C., 600° C. and 700° C. under the following conditions.

Gas composition 1,000 ppm of CO + 667 ppm of C₃H₆ + 250 ppm of NO + 7.3% of O₂ + 6.7% of CO₂ + 5% of H₂O (balance: N₂) Gas space velocity: 50,000 h⁻¹

FIGS. 1 to 6 show the NO_(x) purification percentage (NO_(x) purification percentage for one minute) determined according to the following formula: Purification percentage=[(incoming gas concentration−outgoing gas concentration)÷incoming gas concentration]×100

It is seen from the results shown in FIGS. 1 to 6 that the NO_(x) absorbent obtained by adding Na, K, Cs or Ba to a crystalline ZrO₂ composite oxide enhances the NO_(x) purifying capability at high temperatures with a tendency that the effect of Cs is higher than K, Ba and Na, and K and Ba are almost on the same level and higher than Na (Cs>K≈Ba>Na).

Also, it is seen from comparison with the NO_(x) absorbent of Comparative Examples obtained by adding Na, K, Cs or Ba to γ-alumina that the NO_(x) purification percentage is higher in Examples with any component added and the NO_(x) purifying capability at high temperatures is enhanced by the NO_(x) absorbent of the present invention.

(2) NO_(x) Purification Percentage at 300 to 700° C.

The catalyst of Example 11 obtained by loading Cs, the catalyst of Example 21 obtained by loading Al and Cs, the catalyst of Example 22 obtained by further loading Pt, and the catalyst of Comparative Example 3 obtained by loading Cs on γ-alumina each was evaluated on the NO_(x) purification percentage at 300° C., 400° C., 500° C., 600° C. and 700° C. under the conditions described above. FIG. 7 shows the results.

It is seen from the results shown in FIG. 7 that the catalyst of the present invention is enhanced in the NO_(x) purifying capability even at temperatures lower than 500° C. and the NO_(x) purifying capability is more enhanced by further loading Al.

(3) NO_(x) Purification Percentage after Endurance Heat Treatment

The catalysts of Examples 11, 21 and 22 and Comparative Example 3 each was subjected to an endurance heat treatment of 850° C.×2 hours in a gas atmosphere having the following composition.

Gas composition 1,000 ppm of CO + 250 ppm of NO + 7.3% of O₂ + 6.7% of CO₂ + 5% of H₂O (balance: N₂) Gas space velocity: 50,000 h⁻¹

Subsequently, the NO_(x) purifying capability at 300 to 700° C. was evaluated under the following conditions. FIG. 8 shows the results.

Gas composition 1,000 ppm of CO + 667 ppm of C₃H₆ + 250 ppm of NO + 7.3% of O₂ + 6.7% of CO₂ + 5% of H₂O (balance: N₂) Gas space velocity: 50,000 h⁻¹

It is seen from the results shown in FIG. 8 that the exhaust gas purifying catalyst of the present invention maintains high NO_(x) purifying capability even after the endurance heat treatment.

As verified above, the exhaust gas purifying catalyst of the present invention is excellent in the NO_(x) purifying capability at high temperatures and also in the resistance against sulfur poisoning. This is considered to result because the alkali metal or alkaline earth metal necessary for producing a strong base site is present as ion and therefore, even if bonded to SO₄ ²⁻, can be easily reduced.

INDUSTRIAL APPLICABILITY

As described in the foregoing pages, according to the present invention, an exhaust gas purifying catalyst exhibiting excellent NO_(x) purifying capability under high exhaust gas temperature conditions, and also having high durability, can be provided. 

1. A NO_(x) absorbent comprising a crystalline ZrO₂ composite oxide having supported thereon at least one element selected from the alkali metals and alkaline earth metals, the ZrO₂ composite oxide having the formula: M_(m)Zr_(1-m)O_(n) Wherein M is at least one element of a trivalent rare earth metal selected from Sc, Y, La, Nd, Pm, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu, and a divalent alkaline earth metal selected from Be, Mg, Ca, Sr, Ba and Ra; 0.01<m<0.5 and 1.5<n<2.0.
 2. The NO_(x) absorbent as claimed in claim 1, further comprises aluminum is supported on said ZrO₂ composite oxide.
 3. An absorption reduction-type NO_(x) purifying catalyst comprising the NO_(x) absorbent as claimed in claim
 2. 4. An absorption reduction-type NO_(x) purifying catalyst comprising the NO_(x) absorbent as claimed in claim 2 and a noble metal.
 5. An absorption reduction-type NO_(x) purifying catalyst comprising the NO_(x) absorbent as claimed in claim 1 or claim
 2. 6. An absorption reduction-type NO_(x) purifying catalyst comprising the NO_(x) absorbent as claimed in claim 1 or claim 2 and a noble metal. 